Re: [RFC PATCH 0/5] Add enum_fmt flag for coded formats with dynamic resolution switching

[Tomasz Figa <tfiga@chromium.org>]

On Mon, Jul 15, 2019 at 9:37 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:
>
> On 6/11/19 10:13 AM, Hans Verkuil wrote:
> > On 6/9/19 4:38 PM, Maxime Jourdan wrote:
> >> Hello,
> >>
> >> This RFC proposes a new format flag - V4L2_FMT_FLAG_DYN_RESOLUTION - used
> >> to tag coded formats for which the device supports dynamic resolution
> >> switching, via V4L2_EVENT_SOURCE_CHANGE.
> >> This includes the initial "source change" where the device is able to
> >> tell userspace about the coded resolution and the DPB size (which
> >> sometimes translates to V4L2_CID_MIN_BUFFERS_FOR_CAPTURE).
> >
> > Shouldn't the initial source change still be there? The amlogic decoder
> > is capable of determining the resolution of the stream, right? It just
> > can't handle mid-stream changes.
>
> I've been thinking about this a bit more: there are three different HW capabilities:
>
> 1) The hardware cannot parse the resolution at all and userspace has to tell it
> via S_FMT.
>
> 2) The hardware can parse the initial resolution, but is not able to handle
> mid-stream resolution changes.
>
> 3) The hardware can parse the initial resolution and all following mid-stream
> resolution changes.
>
> We can consider 2 the default situation.

Any particular reason for 2 being the default? I'm especially
wondering about that as most of the drivers actually provide 3.

>
> In case of 1 the SOURCE_CHANGE event is absent and userspace cannot subscribe
> to it. Question: do we want to flag this with the format as well? I.e. with a
> V4L2_FMT_FLAG_MANUAL_RESOLUTION? I think just not implementing the SOURCE_CHANGE
> event (and documenting this) is sufficient.
>
> In case of 3 the format sets the V4L2_FMT_FLAG_DYN_RESOLUTION flag.
>
> What do you think?
>
> Regards,
>
>         Hans
>
> >
> > Regards,
> >
> >       Hans
> >
> >> This flag is mainly aimed at stateful decoder drivers.
> >>
> >> This RFC is motivated by my development on the amlogic video decoder
> >> driver, which does not support dynamic resolution switching for older
> >> coded formats (MPEG 1/2, MPEG 4 part II, H263). It does however support
> >> it for the newer formats (H264, HEVC, VP9).
> >>
> >> The specification regarding stateful video decoders should be amended
> >> to include that, in the absence of this flag for a certain format,
> >> userspace is expected to extract the coded resolution and allocate
> >> a sufficient amount of capture buffers on its own.
> >> I understand that this point may be tricky, since older kernels with
> >> close-to-spec drivers would not have this flag available, yet would
> >> fully support dynamic resolution switching.
> >> However, with the spec not merged in yet, I wanted to have your opinion
> >> on this late addition.
> >>
> >> The RFC patches also adds support for this flag for the 4 following
> >> stateful decoder drivers:
> >>  - venus
> >>  - s5p-mfc
> >>  - mtk-vcodec
> >>  - vicodec
> >>
> >> Maxime Jourdan (5):
> >>   media: videodev2: add V4L2_FMT_FLAG_DYN_RESOLUTION
> >>   media: venus: vdec: flag OUTPUT formats with
> >>     V4L2_FMT_FLAG_DYN_RESOLUTION
> >>   media: s5p_mfc_dec: flag OUTPUT formats with
> >>     V4L2_FMT_FLAG_DYN_RESOLUTION
> >>   media: mtk-vcodec: flag OUTPUT formats with
> >>     V4L2_FMT_FLAG_DYN_RESOLUTION
> >>   media: vicodec: flag vdec/stateful OUTPUT formats with
> >>     V4L2_FMT_FLAG_DYN_RESOLUTION
> >>
> >>  Documentation/media/uapi/v4l/vidioc-enum-fmt.rst   |  7 +++++++
> >>  drivers/media/platform/mtk-vcodec/mtk_vcodec_dec.c |  4 ++++
> >>  drivers/media/platform/mtk-vcodec/mtk_vcodec_drv.h |  1 +
> >>  drivers/media/platform/qcom/venus/core.h           |  1 +
> >>  drivers/media/platform/qcom/venus/vdec.c           | 11 +++++++++++
> >>  drivers/media/platform/s5p-mfc/s5p_mfc_common.h    |  1 +
> >>  drivers/media/platform/s5p-mfc/s5p_mfc_dec.c       | 13 +++++++++++++
> >>  drivers/media/platform/vicodec/vicodec-core.c      |  2 ++
> >>  include/uapi/linux/videodev2.h                     |  5 +++--
> >>  9 files changed, 43 insertions(+), 2 deletions(-)
> >>
> >
>